It is known from the state of the art how to project information in the field of sight of a user, such as, for example, a driver or a pilot, by means of a head-up display, or HUD.
Such head-up displays generally feature a picture generating unit, which provides the information to be represented in the form of an image, an optical module, which permits the beam path through the head-up display as well as a combiner, to superimpose the image over a background view. Additional optical devices such as mirrors and lenses may also be included to guide the image onto the combiner. In some cases, the windshield of a vehicle is used as the combiner. The viewer, who may be the driver of the vehicle, sees an image from the picture generating unit superimposed with the actual surroundings behind the windshield. The attention of the vehicle driver, for example, when driving a motor vehicle, therefore remains focused on that which is occurring in front of the vehicle, while they are able to grasp the information that is projected in the field of vision.
As illustrated in FIG. 1, a common problem with head-up displays is the creation of double images that may result, for example, where the image is projected upon a device such as a windshield that has two or more surfaces or other interfaces between materials having different indices of refraction (e.g. between glass and air). The two surfaces or interfaces may each produce a separate reflection, with the reflections being offset from one another, which may appear as double-images. Double-images that are close enough together may also appear as a blurry image, or one which is out of focus.
One solution to the problem of double-images known from the prior art is to use a wedge-shaped windshield with surfaces that are not substantially parallel to one-another. Such wedge-shaped windshields have drawbacks including production costs and added weight of materials such as glass.
Another existing solution known from the prior art is to use a normal, panel windshield with a coating and with polarized light used for illumination. However, the combination of polarized illumination with a windshield having a coating has the drawback of producing virtual images with relatively low brightness.